Hillary Clinton’s closing argument is all about Donald Trump, as she piled on accusations Monday that the Republican presidential nominee is a threat to national security, in league with Russia and ready to start nuclear war.

The rapid-fire assault — including a lengthy white paper, a new TV ad and a stump speech focused almost entirely on attacking Mr. Trump — served to deflect attention for the FBI reopening an investigation of Mrs. Clinton for using a secret email setup as secretary of state, which sent her campaign into panic mode a week before the election.

At a rally at Kent State University in Ohio, Mrs. Clinton implied that there were political motivations behind the FBI probe into her secret email setup at the State Department and potentially criminal mishandling of government secrets, and quickly pivoted to attacking Mr. Trump.

“I am running against a man who says he doesn’t understand why we can’t use nuclear weapons. He actually said, ‘Then why are we making them?’ ” she said. “He wants more countries to have nuclear weapons — Japan, South Korea, even Saudi Arabia. Imagine nuclear weapons smack in the middle of the Middle East.”

Mrs. Clinton said the most important question facing voters in the presidential election was whether the next president can be trusted with the nuclear arsenal and “literally make life-or-death decisions about war and peace.”

Meanwhile, Mr. Trump, at a rally in Michigan, blasted Mrs. Clinton with allegations that she is a criminal and applauded the FBI for finally catching up with her.

FBI Director James B. Comey informed Congress that the investigation had reopened, but he did not announce criminal charges.

“Hillary is the one who broke the law over and over and over again. We can be sure that what is in those emails is absolutely devastating,” Mr. Trump said. “Hillary is not the victim — the American people are the victims of this corrupt system in every way, and this is your one chance to change it.”

The exchange of allegations capped a presidential campaign that has been dominated by two candidates with high unfavorable ratings fighting over who will be chosen as least popular. Both took aim at their rival’s soft spots: Mrs. Clinton’s trustworthiness and Mr. Trump’s temperament.

In Ohio, Mrs. Clinton also accused Mr. Trump of either being a willing dupe for Russian President Vladimir Putin or conspiring to aid the Kremlin, suggesting the billionaire businessman would betray his own country to reap a profit.

“Putin is a trained intelligence officer from the old KGB. He knows he can use flattery to get into Donald’s head to make Donald the Kremlin’s puppet — and it seems to be working,” she said.

Noting that U.S. intelligence officials have identified Russia as the likely culprit behind email hacks that targeted the Democratic National Committee and the Clinton campaign, she suggested that it was a scheme by Mr. Putin to get Mr. Trump elected.

“So ask yourself, why would Putin be trying to get Donald elected president?” Mrs. Clinton said. “Could it be because of all the nice things Donald has said about him, or could it be the fact that he has promised to adopt pro-Kremlin policies, or maybe because of the extensive business dealing with Russian oligarchs with ties to Putin?”

Democratic campaign strategist Michael Czin said highlighting questions about Mr. Trump’s temperament was a smart tactic at any stage of the campaign, but also a convincing closing argument in the face of a renewed FBI probe.

He said accusing Mr. Trump of threatening nuclear war was not out of bounds.

“It’s over the top, but it’s not over the top for us to say it. It’s over the top for him to say it,” said Mr. Czin, noting that Mr. Trump asked in an interview with MSNBC’s Chris Matthews why the U.S. shouldn’t use nuclear weapons.

He said it put the FBI probe in the context of what’s at stake in the election.

“It helps make the choice clear to voters in the closing days that you have someone who has this fringe view on foreign policy, and that does help,” Mr. Czin said. “Regardless of what’s going on with Comey and the FBI, I think to make the stakes clear now is beneficial. At any point in the campaign, this was an argument that is beneficial.”

The white paper listed more than 70 statements Mr. Trump made about foreign policy issues that Mrs. Clinton, the Democratic nominee, deemed misguided or dangerous, including saying he “loves war” and that North Korean dictator Kim Jong-un deserves “credit” for taking out his rivals.

The campaign hammered home the message that Mr. Trump is “reckless” with a new TV ad that revived the iconic 1964 “Daisy” commercial that warned Republican Barry Goldwater would start a nuclear war.

The Clinton ad featured Monique Luiz, the same actress who at age 3 portrayed a little girl counting the petals she plucks from a daisy, followed by footage of a mushroom cloud rising from a nuclear explosion.

“The fear of nuclear war that we had as children, I never thought our children would ever have to deal with that again, and to see that coming forward in this election is really scary,” Ms. Luiz says in the new ad.

The ad shows TV news personalities discussing Mr. Trump’s repeatedly asking in an interview why the U.S. should not use its nuclear arsenal and a clip of Mr. Trump at a campaign rally saying, “I want to be unpredictable.”

Christopher Borick, director of the Muhlenberg College Institute of Public Opinion, said Mrs. Clinton was targeting a clear weakness of Mr. Trump‘s.

“If this is a game right now of who is going to be least favorable, at the end of the day, this is a strategy that is designed to go down that path — to keep reinforcing his unfavorables, and where he is most unfavorable is on issues like temperament,” he said.

However, the political science professor said that he doubted the accusations of recklessness with foreign policy would fully counter questions raised by the FBI probe.

“She’ll try to do that this week with ads and other means to focus public attention on Trump’s liabilities, which are many,” he said. “The FBI announcement is going to make that hard for a few days, at least in the sense that it is such a big reinforcer of existing concerns, and she has to overcome that.”